This invention relates to a copying apparatus of the kind including a transparent glass platen across which documents for copying are moved, and means for removing electrostatic charges from the platen.
Typical of such copying apparatuses are xerographic copying machines, especially those including an automatic or semi-automatic document handler. In such machines, documents to be copied are moved across the platen by a suitable drive mechanism. In one arrangement, each document is moved from a start position into a stationary exposure position on the platen. After exposure, the document is moved off the platen. In another arrangement, each document is moved continuously across the platen during exposure. In a recirculating document handler, documents are fed successively from a stack of documents, across the platen, and back into the stack. Typical of driving mechanisms for the documents is a friction belt, usually a broad, endless belt of plastics material, which drags the documents across the platen. An example of a document handler of this type is described in U.S. Pat. No. 3,556,512.
The glass from which the platen is usually made is standard soda lime float glass, because it is relatively inexpensive, has good light transmission, and can be made sufficiently optically flat to enable good reproduction of the documents to be copied. It has been found that with certain kinds of document handler, especially high speed ones, a build up of static electrical charge takes place on the platen due to frictional contact between the document and the platen, between the document and drive belt, and between the belt and the platen in those parts of the copying cycle where there is no document between the platen and the belt.
This build up of electrostatic charges gives rise to the problem that there is an increasing tendency for documents to adhere to the platen which may eventually cause a document jam, and an interruption of the copying cycle.
A device for preventing the build up of electrostatic charge on the platen of a copying machine is described in Japanese patent application No. 55-164838. This device includes a conductive brush-like member which is arranged to sweep the underside of the platen so as to remove any build up of charge. The brush-like member is driven by the scanning optics mechanism of the copying machine. In a copying machine not using scanning optics, specific provision would need to be made for driving the brush-like member.
It is an object of the present invention to provide a copying apparatus in which the problem of electrostatic charge build up on the platen is overcome without the need for any additional mechanisms.